Description: In June 2021, the western North American continent experienced an intense heat wave with unprecedented temperatures and far-reaching socio-economic consequences. The magnitude of the heat wave was substantially underestimated by probabilistic weather forecasts for lead times beyond seven days. The record-breaking temperature anomaly coincided with a far northward extending upper-level ridge that was unambiguously linked to the intensity of the heat wave. During the 10 days preceeding the heat wave, the upper-level ridge was continuously fed by air masses originating to a substantial fraction from the lower troposphere that ascended in the West, Central, and East Pacific. We analyze the role of these strongly ascending airstreams, so-called warm conveyor belts (WCBs), for this extreme event using the operational ensemble forecasts from the European Centre for Medium-Range Weather Forecasts. Our analysis illustrates how anomalous WCB activity across the North Pacific - which is also associated with above-normal precipitation at the Meiyu-Baiu-Front - limited the predictability horizon of this extreme event. The results suggest that a mis-representation of WCB-related synoptic activity across the West and East Pacific in the ensemble forecasts results in an erroneous ridge position over the North American continent and a concomitant underestimation of the heat. We conclude that this chain of synoptic events which was essential for the upper-level ridge position and amplitude constituted a predictability barrier for the magnitude of the heat wave.

Description: The characteristic and strongly precipitating cloud band in extratropical cyclones is associated with the so-called warm conveyor belt (WCB), which is a coherent airstream ascending cross-isentropically from the boundary layer into the upper troposphere within two days. The WCB ascent behaviour and associated diabatic heating are influenced by microphysical processes and environmental conditions in the WCB inflow region. The former relies on parametrisation schemes, the latter on initial and/or boundary conditions of thermodynamic variables. Altogether, this introduces uncertainty in numerical weather prediction and ultimately for the evolution of the large-scale mid-latitude flow. Based on a case study from the NAWDEX field campaign, we quantify the relative importance of perturbations to various microphysical processes and WCB inflow temperature and moisture (via modification of sea surface temperature). Thereby, we focus on uncertainty in WCB ascent behaviour, associated precipitation characteristics, as well as properties of the amplifying ridge downstream of the ascent region. To disentangle individual uncertainty contributions, we build a 70-member perturbed parameter ensemble which systematically combines the perturbations, and subsequently perform variance-based sensitivity analysis. Our results suggests that changes to WCB inflow properties most strongly influence WCB ascent behavior, surface precipitation sums, and the ridge amplitude. Yet, the microphysical perturbations locally modify vertical velocity in the WCB and determine the precipitation efficiency, which affects local to meso-scale precipitation characteristics and its spatial distribution. Moreover, the microphysical perturbations are distinctly imprinted in the large-scale flow pattern - albeit to a lesser extent than the perturbations applied to the WCB inflow characteristics.

Description: Dusty cirrus clouds are extended optically thick cirrocumulus decks that occur during strong mineral dust events. So far, they have been mostly documented over Europe associated with dust-infused baroclinic storms. Since today's global operational numerical weather prediction models neither predict mineral dust distributions nor consider the interaction of dust with cloud microphysics, they cannot simulate this phenomenon. We have performed ICON-ART limited-area simulations with 2 km grid spacing to understand and predict the formation of dusty cirrus clouds. Based on these simulations, we postulate that the dusty cirrus forms through a mixing instability of moist clean air with drier dusty air. A corresponding sub-grid parameterization is suggested and tested in the ICON-ART model. Only with help of this special sub-grid parameterization ICON-ART is able to simulate the formation of the dusty cirrus, which leads to substantial improvements in cloud cover and radiative fluxes compared to simulations without this parameterization. A statistical evaluation over six Saharan dust events with and without observed dusty cirrus shows robust improvements in cloud and radiation scores. The ability to simulate dusty cirrus formation removes the linear dependency on mineral dust aerosol optical depth from the bias of the radiative fluxes. This suggests that the formation of dusty cirrus clouds is the dominant aerosol-cloud-radiation effect of mineral dust over Europe. At the IUGG we will present first simulations with the dusty cirrus parameterization in the global ICON-ART model and discuss the occurrence of dusty cirrus in Asia.

Description: Despite the potentially severe impact of atmospheric blocks on surface weather, the correct prediction of blocking lifecycles remains a key challenge in current numerical weather prediction models. Increasing evidence suggests that latent heat release in cyclones, the advection of cold air from the Arctic over the North Atlantic, and associated air-sea interactions over the Gulf Stream are key processes responsible for the onset and persistence of such flow regimes. In order to establish how air mass transformations over the Gulf Stream affect the large-scale flow we focus on an episode between 20 and 27 of February 2019, when an upper-level ridge established over western Europe - accompanied by an intensified storm track in the North Atlantic. To explore the mechanistic linkage between the formation of this block and air-sea interactions over the Gulf Stream, we adopt a Lagrangian perspective, using backward and forward kinematic trajectories. The detailed investigation of the evolution of potential temperature, moisture, and other variables along the trajectories, as well as of surface fluxes, SST, and SST gradient is carried out to examine the nature of the processes involved in the upper-tropospheric flow variability. Determining the exact geographical location of moisture uptakes as well as their environment allows us to link air-sea interaction processes and the dynamical evolution of the flow. Thereby, we address the hypothesis that air-sea interaction processes over the Gulf Stream, in particular during CAOs, are of fundamental importance for the maintenance of favorable conditions for cyclone intensification and the formation of European blocking.

Description: Various methods have been developed to characterize the large-scale atmospheric circulation, e.g., by classifying the flow into so-called weather regimes or circulation types. We introduce a novel conceptual framework to quantify how relevant frequency changes of such weather regimes are for understanding climate change signals in precipitation. For every regime, a spatially varying parameter γ corresponds to the ratio of the contribution from regime frequency changes to the climate change signal of precipitation to the contribution of regime intensity changes. Conceptual considerations show that γ is (i) proportional to the relative change of the regime frequency, (ii) proportional to the regime-specific anomaly of precipitation, and (iii) inversely proportional to the climate change effect on regime intensity. The combination of these independent and competing factors makes the study of γ interesting and insightful. As a specific example application of this framework, we consider a 7-category weather regime classification in the North Atlantic-European sector and large ensemble simulations with the CESM1 climate model under the RCP8.5 emission scenario for the periods 1990-1999 and 2091-2100. Considering γ for surface precipitation P in this simulation setup reveals, as the main results, that γ values are typically less than 0.1 and therefore, to first order, frequency changes of regimes are not relevant for explaining climate change signals in P, and that the main reason for the generally low values of γ are the comparatively weak regime frequency changes and the limited skill of the regime classification in stratifying precipitation in particular over continental Europe.

Description: Weather regimes govern an important part of the sub-seasonal variability of the mid-latitude circulation. Due to their role in weather extremes and atmospheric predictability, regimes that feature a blocking anticyclone are of particular interest. This study investigates the dynamics of these ''blocked'' regimes in the North Atlantic-European region from a year-round perspective. For a comprehensive diagnostic, wave activity concepts and a piecewise potential-vorticity (PV) tendency framework are combined. All occurrences of four types of blocked regimes (namely Atlantic Ridge, European Blocking, Scandinavian Blocking and Greenland Blocking) during the 1979-2021 period of ERA5 reanalysis are considered. Distinct differences between blocked regimes are found in their wave activity characteristics after regime onset: Greenland Blocking is associated with a suppression of wave activity flux, whereas Atlantic Ridge and European Blocking are associated with a northward deflection of the flux without a clear net change. During onset, the envelope of Rossby wave activity retracts upstream for Greenland Blocking, whereas the envelope extends downstream for Atlantic Ridge and European Blocking. From the piecewise PV tendencies perspective, the dynamics governing regime onset exhibit a large degree of similarity. Most strikingly, all blocked regimes exhibit very similar (intra-regime) variability: a retrograde and an upstream pathway to regime onset, dominated by nonlinear PV eddy fluxes and linear Rossby wave dynamics, respectively. This dynamics-centered variability does not merely reflect that associated with seasonality or different types of regime transitions, but appears to be of more fundamental, dynamical significance than phenomenologically defined variability.

Description: While models often struggle to directly predict extreme events in the subseasonal range, especially those characterised by small spatial scales such as extreme precipitation, prospects are better for predicting large-scale circulation patterns. Fortunately, many extreme events are closely coupled to the large-scale flow: for precipitation, large-scale wave activity determines moisture transport and availability, especially in the mid-latitudes where frontal rainfall is dominant. We present a framework for identifying the precursor patterns that ‘set the scene’ for extreme events, and using them to augment direct model output to produce more skilful hybrid forecasts. Implementation and extension of this framework is supported by a new open-access Python package, Domino, which reduces such analyses to only a few lines of code. Specifically we consider the predictability of European regional daily rainfall extremes at lead times of several weeks. We will discuss how using information about large-scale precursors, in combination with direct model output, allows extra skill to be extracted from our existing models, and allows us to make the most of the high dimensionality and high data volumes produced by modern forecast systems. We will discuss the potential of this flexible framework to be applied to other geographical regions, different kinds of extreme events and to different time-scales, and plans for a semi-operational implementation of this approach using the IFS forecast model.

Description: The intensification of extratropical cyclones is fuelled by the conversion of available potential energy (APE) into kinetic energy by baroclinic instability. While traditional metrics of APE are only available as global integrals and are typically used to explain the maintenance of the general circulation on global scales, APE is converted locally in the ascending and descending principal airstreams of a baroclinic wave. Here, we aim to study local APE conversion within cyclones. To this end, we apply a recently introduced diagnostic of APE, which is exact, defined locally for each air parcel, and readily obtainable from model data. Using the Lagrangian properties of this diagnostic, we clarify the role of coherent air streams for APE conversion during cyclone intensification for an idealized baroclinic wave simulation and a climatology of rapidly intensifying Gulf Stream cyclones in ERA-5. Our findings reveal an intricate balance of baroclinic APE generation and reduction. While the vertical motion within the cyclone leads to a net reduction of APE, locally, APE is also generated. Moreover, latent heat release increases the APE available for baroclinic conversion within coherent air streams, aiding cyclone intensification. The analysis of Gulf Stream cyclones highlights the detrimental effects of surface sensible heat fluxes from the Gulf Stream current for cyclone intensification. This work demonstrates the applicability of the local APE framework to investigate synoptic dynamics and provides the basis for further analysis of the maintenance of favorable conditions for cyclone intensification within storm tracks.